Scientific grade silicon CCD detectors offer substantial improvements in performance for detection of photons in the ultraviolet (100-300 nm) range of the electromagnetic spectrum. In particular, back-illuminated CCDs constructed using delta-doping technology and that have been passivated exhibit at least 30% quantum efficiency (QE) over that entire range, compared to the ˜5-10% achieved by microchannel plates that have been previously flown in space missions such as Galex. However, the performance of these delta-doped CCDs is limited due to the inherent reflectivity of the silicon itself Anti-reflection coatings can, in principle, be applied to the silicon detector to improve the performance up to quantum efficiencies as high as 70%. However, the interface and film quality is critical for this theoretical performance to be achieved. Conventional sputtering and thermal evaporation techniques can be used to deposit films such as HfO2, MgF2, and MgO that should behave as anti-reflection coatings. However, recent experiments with these processes show that the film and interface quality achieved by these techniques are insufficient to achieve the desired quantum efficiency.
Transient Charging of Silicon Surfaces at High Illumination Intensities
High carrier concentrations, for example carrier concentrations that can be induced by intense illumination such as a laser pulse, can lead to many body effects, band flattening, and reduction in minority carrier lifetime near the surface. Despite the exceptionally high charge density in the delta-doped layer, these effects may lead to nonlinearities in the detector response. In general, the quantum efficiency observed at high illumination levels may be lower than the quantum efficiency observed at significantly lower illumination intensities. However, hot carriers produced at such high illumination levels should not induce permanent changes to the detector.
Degradation of Silicon Surfaces by Exposure to UV Radiation
At relatively low intensities, UV radiation can damage the Si—SiO2 interface by hot carrier degradation of the oxide and consequent formation of interface traps, as described by U. Arp et al., “Damage to solid-state photodiodes by vacuum ultraviolet radiation,” Journal of Electron Spectroscopy and Related Phenomena, 144-147: 1039-1042 (2005) and by P-S. Shaw et al., “Stability of photodiodes under irradiation with a 157-nm pulsed excimer laser,” Applied Optics, 44(2): 197-207 (2005). Trap formation is cumulative, and potentially irreversible, which is one reason why surface and interface passivation technologies that rely on initially low defect densities, such as thermally grown oxides and hydrogen passivation, may not remain stable under UV illumination.
At higher intensities, such as those achievable with pulsed laser sources, a single laser pulse may carry enough energy to cause extremely rapid melting and recrystallization of the surface under nonequilibrium conditions, as described by I. Luk{hacek over (s)} et al., “Study of Excimer Laser Induced Melting and Solidification of Si by Time-Resolved Reflectivity Measurements,” Applied Physics A, 54: 327-333 (1992), by R. {hacek over (C)}erný et al., “Nonequilibrium Solidification of Monocrystalline Si Induced by ArF-Excimer-Laser Irradiation,” Thermochimica Acta, 218: 173-182 (1993) and by T. Scheidt et al., “Ultraviolet pulse laser induced modifications of native silicon/silica interfaces analyzed by optical second harmonic generation,” Journal of Applied Physics, 100, 023118 (2006).
Nonequilibrium melting/recrystallization of the silicon surface can occur upon exposure to excimer laser pulses above an energy threshold of ˜0.4 J/cm2. Scheidt et al. measured a damage threshold using a nonlinear optical technique that is sensitive to interface trap density. According to Scheidt et al., the damage threshold is based on peak intensity, so even if the average intensity over the beam is lower than the threshold, they still observe damage at the center of the beam. The damage threshold corresponds to melting and recrystallization induced by single-pulse exposures. However, multiple subthreshold pulses are observed to create interfacial traps, with a cumulative effect on the Si—SiO2 interface. Assuming that the local temperature required to melt the surface is the same as the bulk silicon melting temperature (approximately 1410 C), then subthreshold exposures may cause high enough local temperatures to break bonds in the oxide. Silicon-hydrogen bonds can be broken at temperatures in the vicinity of 400-450 C, and SiO2 decomposition at the silicon surface occurs at temperatures above 800 C. Because the decomposition of SiO2 at the interface involves a chemical reaction with silicon at the interface, the chemical stability of the interface is also a potential concern under conditions of intense illumination.
At still higher intensities, laser ablation occurs, and the surface literally boils and explodes, leaving behind a crater, as described by Q. Lu et al., “Theory analysis of wavelength dependence of laser-induced phase explosion of silicon,” Journal of Applied Physics, 104, 083301 (2008).
With respect to devices having a delta-doped layer, there are at least three potential damage mechanisms relating to high UV illumination fluxes that one should consider. These include UV illumination-induced modifications of the Si—SiO2 interface (Scheidt et al.), UV illumination-induced oxidation of the interface as described by T. E. Orlowski, D. A. Manteli, “Ultraviolet laser-induced oxidation of silicon: The effect of oxygen photodissociation upon oxide growth kinetics,” Journal of Applied Physics, 64(9): 4410-4414 (1988), and UV illumination-induced surface melting and recrystallization as described by Luke{hacek over (s)} et al. and {hacek over (C)}erný et al.
Uv-Induced Formation of Traps at the Si—SIO2 Interface and in the Oxide
Unlike other surface passivation technologies, delta-doped CCDs exhibit excellent stability when only a native oxide is present on the surface. Moreover, the sharply-peaked electronic potential at the delta-doped layer serves as a tunnel barrier to suppress the injection of surface-generated dark current into the bulk silicon comprising the minority carrier collection volume of the detector. Consequently, for low illumination intensities, UV-induced trap formation is not expected to be a significant threat to the stability of the delta-doped surface. This is in sharp contrast with chemisorption charging, which is vulnerable to permanent band-flattening due to accumulation of interface and oxide trapped charge, enhanced surface-generated dark current due to trap formation at the Si—SiO2 interface, and hot-carrier induced degradation of chemisorbed charge.
Under high photon fluxes, dynamic charging of the oxide may take place, as the surface is flooded with hot electrons and hot holes. In particular, different lifetimes of electron and hole traps in the oxide contribute to dynamic charging effects. These dynamic effects depend on materials and coating methods, including possible enhancement in thicker coatings due to the larger volumes involved. High quality, low defect oxides do not ensure long-term stability, as UV-induced damage is known to cause the formation of traps. Because of the high, localized charge in the delta-doped layer, the delta-doped surface is buffered against dynamic charging.
UV-Induced Chemical Reactions at the Interface
In delta-doped, n-channel CCDs and CMOS imaging arrays, the delta-doped surface comprises a sheet of dopant atoms typically located only 1-2 nm below the Si—SiO2 interface. At these length scales and for high-intensity illumination, UV-induced chemical reactions are a potential concern, depending on the illumination intensity and the ambient environment. UV laser irradiation is known to cause oxidation of the silicon surface, especially at photon energies sufficient to cause photodissociation of oxygen, as described by Orlowski et al. Orlowski et al. compared irradiation at 193 nm and 248 nm, and showed that photodissociation of oxygen at the shorter wavelengths dramatically increases the oxidation rate. The highest laser intensities in the Orlowski study reached the melting/recrystallization threshold.
Melting/Recrystallization of the Silicon Surface
According to Luke{hacek over (s)} et al., the depth to which melting occurs is sufficient to engulf the delta-doped layer. In some instances, an excimer laser can be powerful enough to induce melting/recrystallization of the surface when focused to subpixel spot sizes.
The ultraviolet represents an extremely important region of the electromagnetic spectrum as it has a critical role in a wide variety of scientific, commercial, and government applications. For example, ultraviolet spectroscopy can be utilized to study planetary atmospheres to determine whether basic elements for life such as oxygen, nitrogen, and hydrogen are present. A UV spectrometer can also detect evidence small quantities of complex organic molecules, (e.g., tholins), from the UV reflectivity of the surface of an icy moon providing information on how prebiotic chemistry takes place on extraterrestrial bodies. High sensitivity astronomical observations in the UV regime could enable the observation of faint emission from the baryons that form the intergalactic medium, which likely represent 50% of the detectable baryonic mass in the universe. Recent measurements enabled by the first all sky UV survey mission, GALEX, have uncovered a startling comet-like tail behind a red giant star that is streaking through space at nearly 300,000 miles per hour. This phenomenon is unique and can only be observed in the UV, and has now provided a means to characterize how stars can die and ultimately seed new solar systems through the shedding of carbon, oxygen, and other elements. Extreme UV lithography is utilized to pattern the finest features of the latest generation of semiconductor devices. UV laser inspection and imaging is therefore critical to identify defects in the fabrication process to maximize yield and reduce cost in this highly competitive industry. UV imaging has also recently been used in medical imaging to study how caffeine affects calcium ionic pathways in the brain. Rockets produce significant UV emission due to the production of excited nitrogen oxide species in their plumes. While infrared imaging is clearly an important anti-missile defense technology, UV can offer significant advantages in this application due to the ability to observe even in direct sunlight using “solar-blind” imaging. Bite marks can be readily observed and identified in forensic investigations since human saliva (wet or dry) shines brightly under UV illumination. Bruises are also evident for many days in UV after they have disappeared to the naked eye. As this only represents a small fraction of the real world applications of UV detectors, there is clearly strong motivation to have detectors with the highest possible sensitivity.
Unfortunately, despite this wide range of applications, scientific imaging in the ultraviolet is extremely difficult because the technology for sensing UV light is substantially limited by the quantum efficiency of available detectors and the transparency of optical coating materials. Many materials strongly absorb Near and Far UV light such that thicknesses of 20 nanometers or less are completely opaque. Thus, the UV throughput of an instrument is highly sensitive to impurities on, or contained in, any of the optical elements or the detector itself The absorption depth of UV photons is also very short, making collection of photo-induced current difficult with traditional materials. Compounding this problem is that many important sources of UV light are faint, so maximizing detector sensitivity is critical to unlock the true potential of UV imaging for the above applications.
TABLE 1Typical QuantumExample of CurrentEfficiency (155-300 nm)Use in AstronomyCs2Te Microchannel~10% or lessGALEX Space TelescopePlatesSilicon CCD coated~15-25% or lessCassini ISS, Hubblewith LumogenSpace Telescope
Table 1 outlines two examples of ultraviolet sensitive detectors that are currently in use in space missions along with their typical quantum efficiencies in the near and far UV. Although new classes of III-V Nitride materials based on MBE or MOCVD grown GaN or AlGaN hold significant promise for future generations of UV detectors, they are expensive and extremely difficult to grow at sufficient quality to fabricate into imagers with low dark current and high responsivity. Silicon based charge coupled devices (CCDs), however, are based on the same fabrication technologies utilized in the semiconductor industry and are widely used in commercial imaging applications such as digital cameras. This makes silicon CCDs cheap to manufacture with the added benefit that pixel design and layout are both extremely flexible. CMOS (Complementary Metal Oxide Semiconductor) based imagers are also widespread, and their use in commercial and scientific applications has been accelerating over the last few years due to the rapid progress of the overall CMOS industry. Unfortunately, the native oxide that naturally forms on silicon causes unfavorable distortions in silicon's electronic band structure in the near surface region. This leads to the capture of UV produced photoelectrons in surface traps and thus very poor response below 300 nm for silicon imagers. This can be overcome though a combination of techniques known as back illumination and back surface passivation.
Prior art methods, such as chemisorption and ion implant/laser anneal, do exist to passivate the back surface of silicon CCDs. However, these methods have limitations in that they either do not achieve 100% internal quantum efficiency, have undesirably high dark current, and/or are subject to hysteresis and stability issues due to adsorption of oxygen and other gases in the environment on the surface of the CCD. There are also issues relating to the use of antireflection coatings with such passivation systems.
There is a need for systems and methods that can prevent damage to delta-doped back surface illuminated CCD photodetectors designed to operate in the UV region of the electromagnetic spectrum.